The long term goal of this revised project is to elucidate the site and mechanism of action of FSH on the seminiferous epithelium in the rhesus monkey, a representative primate. Hormonal regulation of spermatogenesis exhibits significant species differences and an appreciation of the mechanisms that control the seminiferous tubule in the rhesus monkey may have direct relevance to understanding the regulation of human fertility and the treatment of male infertility. The present proposal focuses on the recent finding from the principal investigator's laboratory that FSH exerts a selective action on the seminiferous tubule of the rhesus monkey to amplify the yield of differentiated spermatogonia. Four specific aims will be addressed: 1) To determine the cellular site of the action of FSH that underlies the amplification in yield of differentiated spermatogonia. 2) To determine the phase of the spermatogonial cell cycle during which FSH exerts its action to amplify the yield of differentiated spermatogonia. 3) To determine whether suppression of apoptosis is the mechanism that underlies the FSH induced amplification of differentiated spermatogonia. 4) To determine whether other postulated actions of FSH (in particular, the stimulation of the population of renewing stem spermatogonia) contribute to the role of this gonadotropin in stimulating spermatogenesis in primates. The adult male rhesus monkey rendered hypogonadotropic by chronic GnRH antagonist treatment (chemically hypophysectomized model) will be the principle experimental paradigm. Hormone replacement will be achieved using Sc testosterone containing Silastic capsules and iv infusions of recombinant hFSH and hLH. For this purpose, GnRH suppressed monkeys will be implanted with indwelling iv catheters, fitted with nylon jackets, and maintained in remote sampling cages which allow continuous access to the venous circulation without tranquilization and without restraint. Morphological analyses of the effects of FSH on the seminiferous tubule will be conducted using standard stereological methods. In situ hybridization with S35-labeled riboprobes to the cDNA of the rat FSH receptor and rat LH receptor to identify the cellular location of the mRNAs encoding these receptors in the monkey testis. In situ DNA 3' end labeling will be used to identify cells undergoing DNA fragmentation which is indicative of apoptotic cell death. Surgical procedures will be conducted using standard aseptic technique with appropriate anesthesia and post surgical analgesia.